The present invention relates to a spindle device adapted to generate a high pressure fluid lubrication film as the rotor rotates, supporting the rotation of the rotor by this fluid lubrication film and, in particular, to a spindle device using a liquid such as water, and coolant liquid as lubrication fluid for such a dynamic pressure bearing.
JP 6-241222 A, JP 6-249236 A, JP 7-19236 A, etc. disclose spindle devices in which the rotation of a main shaft is supported by a dynamic pressure bearing. Such spindle devices are equipped with a housing fixed to a main shaft head or the like of a machine tool, a main shaft connected to a driving unit for rotation, and a radial dynamic pressure bearing and a thrust dynamic pressure bearing each of which is constructed of a rotary side member and a stationary side member opposed to each other through the intermediation of a predetermined bearing gap and adapted to rotatably support the main shaft with respect to the housing, with a dynamic pressure generating groove with a depth of approximately 10 to 15 xcexcm being formed in a predetermined pattern in the rotary side member of each dynamic pressure bearing.
In a spindle device constructed as described above, lubrication fluid existing in the bearing gaps of the dynamic bearings is pressurized as the main shaft rotates, and the main shaft is levitated by a high pressure fluid lubrication film, its rotation being supported in this state. Thus, only a slight rotational resistance is offered to the rotation of the main shaft, and substantially no vibration due to the rotation is generated, so that this spindle device is advantageously capable of being used with the main shaft being rotated at a high speed of 10,000 rpm or more.
In this spindle device, the lubrication fluid is pressurized as the main shaft rotates, so that, if the bearing gap between the rotary side member and the stationary side member of each dynamic pressure bearing is excessively large, it is impossible to sufficiently increase the pressure of the lubrication fluid in the bearing gap, resulting in deteriorations in the load capacity and rigidity of the main shaft with respect to external load. In view of this, in the above-described conventional spindle device, the bearing gap is set to approximately several xcexcm, making it possible to sufficiently pressurize the lubrication fluid even during low speed rotation of the main shaft.
The lubrication fluid pressurized in the bearing gap of each dynamic pressure bearing may be a gas such as air or a liquid such as water and a coolant for machine tools. Since a gas is more compressible than a liquid, the load capacity and rigidity of the main shaft with respect to load are improved by using a liquid as the lubrication fluid instead of a gas.
In a spindle device for use in a machine tool or the like, a large load may be applied to the forward end of the main shaft from a radial direction perpendicular to the axial direction. In this case, a large bending moment is applied to the main shaft. And, to support the rotation of the main shaft against this bending moment, it is effective to make the bearing gap still smaller or make the radial dynamic pressure bearing long in the axial direction.
However, in either case, rotation of the main shaft causes a large shearing force to be applied to the lubrication fluid. Further, in the case in which a liquid (hereinafter referred to as xe2x80x9clubricantxe2x80x9d) that is inferior to a gas in compressibility is used as the lubrication fluid, the shearing force applied is still larger, and the load on the motor for rotating the main shaft increases. Further, the main shaft undergoes thermal expansion due to shearing frictional heat, resulting in a deterioration in the work machining precision of the machine tool. Further, the lubricant supplied to the dynamic pressure bearings attains high temperature, so that an attempt to recover the lubricant from the dynamic pressure bearings for re-supply could involve the danger of heating the main shaft.
To avoid such problems, it is necessary, for example, to provide a water jacket around each stationary side member to thereby directly cool the dynamic pressure bearings, or to cool the lubricant discharged from the bearing gaps outside the housing before supplying it again to the bearing gaps, with the result that the dynamic pressure bearings have to be rather complicated in structure and increased in size.
The present invention has been made in view of the above problems in the prior art. It is an object of the present invention to provide a spindle device endowed with a sufficient load capacity for bending moment and capable of mitigating the power loss and the heating of the main shaft attributable to the radial dynamic pressure bearing.
In order to attain the above-mentioned object, a spindle device using a dynamic pressure bearing of the present invention is characterized by including: a housing; a spindle rotor one end portion of which protrudes from the housing; a pair of radial dynamic pressure bearing portions provided on the spindle rotor at a predetermined axial interval and rotatably supporting the spindle rotor with respect to the housing; a supply flow passage for guiding lubricant to bearing gaps of the radial dynamic pressure bearing portions; a first cooling chamber provided between the pair of radial dynamic pressure bearing portions so as to circumferentially surround the spindle rotor and communicating with the bearing gaps of the radial dynamic bearing portions; and an air inlet adapted to introduce atmosphere outside the housing into the first cooling chamber.
This technical measure, in which the pair of radial dynamic pressure bearing portions for supporting the rotation of the spindle motors is provided at predetermined axial intervals, provides sufficient rigidity with respect to any bending moment applied to the spindle rotor. Further, since the first cooling chamber surrounding the spindle rotor is formed between the pair of radial dynamic pressure bearing portions, no great shearing force is applied to the portion of the spindle rotor where this first cooling chamber is formed, making it possible to mitigate heat generation and power loss of the spindle rotor.
Further, since the bearing gap of each radial dynamic pressure bearing portion and the first cooling chamber communicate with each other, the lubricant supplied to the bearing gap of each radial dynamic pressure bearing portion and pressurized in the bearing gap is directly ejected into the first cooling chamber, into which atmosphere outside the housing is introduced through the inlet, with the result that the first cooling chamber is maintained at a pressure substantially equal to the atmospheric pressure. Thus, when the lubricant pressurized to a very high pressure in the bearing gap is ejected into the first cooling chamber, which is substantially at the equal pressure as the atmospheric pressure, and the lubricant is ejected as fine droplets in an atomized state, so that the interior of the first cooling chamber is cooled due to heat of vaporization, whereby the spindle rotor surrounded by the first cooling chamber can be positively cooled.
The lubricant ejected into the first cooling chamber is turned into droplets to be eventually discharged to the exterior of the housing. If the droplets adhering to the spindle rotor were allowed to rotate with the spindle rotor, that would result in an accordingly larger load on the rotation of the spindle motor, resulting in power loss. Thus, from this point of view, it is desirable to impart water repellency to the peripheral surface of the spindle rotor facing the first cooling chamber, thereby preventing lubricant from being rotated together with the spindle rotor.
Further, in an aspect where the spindle device of the present invention is actually employed, a thrust dynamic pressure bearing portions for regulating a movement in an axial direction of the spindle motor may be provided. From a viewpoint of miniaturization and satisfactory balance of the spindle device, it is preferable that the spindle device further includes a pair of thrust dynamic pressure bearing portions provided so as to axially sandwich the pair of radial dynamic pressure bearing portions to establish communication between bearing gaps of the thrust dynamic pressure bearing portions and the bearing gaps of the radial dynamic pressure bearing portions adjacent thereto. In this case, it is effective that each radial dynamic pressure bearing portion is equipped with dynamic pressure generating grooves for pressurizing lubricant toward the bearing gap of the thrust dynamic pressure bearing portion and pressurization discharge grooves for pressurizing the lubricant toward the above-mentioned first cooling chamber. With this arrangement, a part of the lubricant supplied to the radial dynamic pressure bearing portions can be reliably ejected into the first cooling chamber, making it possible to cool the spindle rotor. Further, it is also possible to prevent the atmosphere introduced into the first cooling chamber from outside the housing from flowing into the bearing gaps of the radial dynamic pressure bearings, thereby avoiding the danger of foreign matter entering the radial dynamic pressure bearing portions.
Furthermore, in order to prevent intrusion of dust into the housing from outside, it is desirable to provide a labyrinth seal portion between the spindle rotor and the housing, to thereby hermetically seal the atmosphere in the housing from the exterior thereof. In this case, when water repellency has been imparted to the labyrinth seal portion, it is possible to prevent coolant or the like from entering the housing through the labyrinth portion by capillary action even if coolant or the like is poured from outside onto the labyrinth seal portion, thus enhancing the sealing property for the housing.